This invention relates generally to cellular telephone communications, and more particularly to a system and method for CDMA receivers to adaptively change the bias in the sequential detection of a code, to account for the effects of fading.
Spread spectrum communication techniques allow communicating users to operate in noisy radio frequency (RF) spectrums, and are especially effective against narrow-band interferers. Spread spectrum communications can be effected at relatively low power spectral densities, and multiple users can share the same frequency spectrum. Further, receivers can be designed to protect against multipath. These system characteristics encouraged early development of the technology by the military.
Common forms of spread spectrum systems include chirp, frequency hopping, and direct sequence or pseudonoise (PN). The chirp system transmits an impulse signal in the time domain that is spread monotonically in the frequency domain. A receiver converts the spread frequency signal back into an impulse signal. These frequency-spread impulse signals have applications in radar, for the pulse position modulation of information, or both, such as the R.sup.3 transponder developed by General Dynamics, Electronics Division in the 1970s. Frequency hopping systems communicate by synchronizing users to simultaneously change the communication frequency.
Direct Sequence systems spread a digital stream of information, typically in a quadriphase modulation format, with a PN code generator, to phase shift key modulate a carrier signal. The pseudonoise sequence of the PN code generator is periodic, and the spread signal can be despread in a receiver with a matching PN code. Direct Sequence systems have excellent immunity to noise. The PN codes used typically permit a large number of users to share the spectrum, with a minimum of correlation between the user's PN codes. However, Direct Sequence system require large RF bandwidths and long acquisition times.
The IS-95 standard defines key features of the so-called second generation code division multiple access (CDMA) communication system, a type of Direct Sequence spread spectrum modulation. The IS-95 system communicates information from the base station to the mobile stations through a series of traffic channels. These traffic channels are transmit and receive information, i.e. digitized audio signals, spread with a traffic channel PN code, unique to each mobile station. Using this precise timing and phase information derived from the pilot channel, the mobile station is able to acquire a setup channel, and eventually, the overall System Time. With this System Time, the mobile station is able to differentiate between base stations and synchronize the demodulation circuitry with sufficient accuracy to recover the received traffic channel message.
A third generation, wideband CDMA (W-CDMA) system is in development as described in "Wideband-CDMA Radio Control Techniques for Third Generation Mobile Communication Systems", written by Onoe et al., IEEE 47.sup.th Vehicular Technology Conference Proceedings, May 1997, that may have global applications. Instead of a pilot channel, the W-CDMA system has a broadcast, or perch channel. Each timeslot, or slot of the broadcast channel consists of a series of time multiplexed symbols. A long code masked, or special timing symbol segment uses just a short code to spread one symbol of known information. This segment allows a mobile station to acquire system timing information immediately after turn-on. The pilot, or reference symbols are similar to the IS-95 pilot channel. In one proposal, 4 reference symbols, with each symbol being 2 bits, are spread with a long code and a short code. The reference symbol information and the short code are known by the mobile stations. The long code is unique to each base station, so that timing information is refined, once the long code is known (the base station is identified). Other combinations of reference, special timing, and data symbols are also possible.
The W-CDMA system also includes several traffic channels to communicate information, such as a digitized voice or data. The traffic channel predominately includes information, but may also include a reference symbol segment. For example, at a data rate of 32 kilosymbols per second (ksps), a slot could include 4 pilot symbols and 16 information symbols. Precise timing information can be derived during the reference symbols segment of the traffic channel message, but not during the information segments.
Sequential detection techniques are well known for determining the code used to spread information, from a group of candidate codes. The advantage of such a technique is the relatively quick rejection of false candidate codes. Since all but one of the candidate codes is false, the quick dismissal of false codes greatly speeds the acquisition of the correct code. A bias value is added to the integrated despreading results. The biased output follows a different (positive) slope when the correct code is used. When an incorrect code is used the biased output follows a negative slope. When the biased output results falls below a minimum threshold, it is determined that the current code is incorrect, and a new candidate code is selected. When the biased output exceeds a maximum threshold, the candidate code must be the spreading code.
The sequential detection system works very well in controlled environments where the signal to noise ratio of the received signal is known. Then, the bias value remains constant. However, the bias value required for sequential detection is constantly changing in many real-world applications, such as in cell phone communications. The signal to noise ratio of the signal being received is constantly changing as the receiver moves, and as the number of communications in the system varies.
Co-pending patent application, Ser. No. 09/015,424, invented by Kowalski et al. entitled SYSTEM AND METHOD FOR CDMA CHANNEL ESTIMATION, attorney docket no. SMT 301, filed on Jan. 29, 1998, and assigned to the same assignees as the instant application, discloses a procedure for using timing, derived from the perch channel in a wideband CDMA system, to despread and demodulate the traffic channels. Although the system simplifies the operation of the traffic channel, no particular system for simplifying the search for a long code is presented.
It would be advantageous if a CDMA receiver design could simplify the task of determining the long code being used by a base station to code a transmitted message.
It would be advantageous if a bias could be calculated for use in the sequential detection of CDMA type signals when the signal to noise ratio of the received signals is ever-varying.
In a system where the signal to noise ratio of the received signals varies, it would be advantageous if the bias value could be calculated in response to the varying signal to noise ratio. It would be advantageous if a spreading code could be determined through sequential detection with the use of the varying bias value.
Accordingly, in a wideband code division multiple access (W-CDMA) communication system including a base station transmitting a channel of information spread with a unique first long code, a sequential searching receiver with an adaptive threshold bias is provided. For reasons explained below, the adaptive sequential detection technique described herein only works when a periodic pattern of bits can be established for reference. These reference bits, known as a long code masked symbol, must be of known value, i.e., all 1s, and must not be spread with the first long code. The W-CDMA system provides such reference bits.
The receiver comprises a short code matched filter (MF) having an input to receive the transmitted channel of information spread with the first long code. The short code MF has an output to provide an output peak signal corresponding to the first long code masked symbol. The receiver also includes a timing and code management (TCM) circuit having an input operatively connected to the short code matched filter output. The TCM circuit computes a bias based on the amplitude of the first long code masked symbol peak output, and computes the timing relationships from the interval between long code masked symbol peak outputs. The TCM circuit has an output to provide the bias.
The receiver also includes a searcher unit having a first input to accept the transmitted spread first long code, and a second input operatively connected to the TCM circuit output. The searcher unit correlates the first long code against candidate long codes, integrating the correlation results, and using the bias to adjust the integrated correlation results. The searcher compares the biased integrated correlation results to pass/fail criteria. The searcher generates a selected candidate code to despread the first long code, when the pass criteria is fulfilled. In this manner, the searcher compensates for the effects of fading in the transmitted signal, and permits the use of a sequential detection technique to search for a spreading code.
A method for a mobile station receiver to acquire the base station through the sequential detection of the first long code from a plurality of long code candidates is also provided. The method comprises the steps of:
a) despreading the received channel with a candidate long code; PA1 b) continuously integrating the output of the despreading process of Step a); PA1 c) adding a bias to the integrated output of Step b); PA1 d) establishing an acceptance threshold; PA1 e) comparing the biased integrated output to the acceptance threshold; and PA1 f) selecting a candidate long code as the first long code in response to the comparison of Step e), whereby the base station is acquired. PA1 g) establishing a rejection threshold; PA1 h) comparing the biased integrated output to the rejection threshold; PA1 i) in response to the comparison to the rejection threshold, rejecting the candidate long code as the transmitted long code; and PA1 j) selecting a new long code candidate. PA1 a.sub.1) measuring the amount of fade in the received channel of information; and
In some aspects of the invention, further steps follow Step c), of:
Typically, the channel of information spread with the first code is subject to fading in transmission from the base station to the mobile station receiver, then further steps follows Step a), of:
In addition, Step c) includes adjusting the bias in response to the amount of measured fade, whereby the number of incorrect long code acceptance/rejection selections are reduced. In some aspects of the invention, Step a.sub.1) measures the amplitude of the despread transmitted channel during the long code masked symbol, and Step c) includes adjusting the bias value in response to the amplitude measured during the long code masked symbol, whereby the bias value is adjusted for the effects of fading.